1. Field of the Invention
This invention relates to CVD (chemical vapor deposition) and more particularly to a method and an apparatus for laser-induced CVD, in which a reactive gas is decomposed photochemically by a laser beam having an extremely high density of photons thereby to deposite a thin film on a substrate.
2. Description of the Prior Art
A laser-induced CVD method is described, for example, by P. K. Boyer et al. in Appl. Phys. Lett., vol. 40, 1982, pp. 716-718.
Referring to FIG. 1, there is illustrated the principle of a laser CVD method in a simplified conceptional front view. A substrate 2 is placed on a susceptor 3 in a reactive gas. A laser beam 1 is set parallel to the top surface of the substrate 2 and is spaced above about 0.5 to a few millimeters therefrom. The reactive gas is decomposed photochemically by the laser beam 1 and deposites a thin film on the substrate 2.
The thin film is deposited on the substrate 2 without direct laser beam irradiation to the substrate 2 as described above, and this is because excited species formed by photochemical decomposition of the reactive gas in the beam pass region are diffused toward the substrate 2.
More specifically, for example, a silicon film is deposited on a substrate in a reactive gas of SiH.sub.4 by utilizing an ArF excimer laser (wavelength: 193 nm) or a KrF excimer laser (wavelength: 248 nm) as a light source.
In this case, there is a question in the relation between the absorption spectrum of the SiH.sub.4 gas and the spectrum of the excitation light. Generally, in order to cause a photochemical reaction in a reactive gas, it is necessary to coincide the spectrum of the excitation light with the absorption spectrum of the reactive gas. In other words, a photochemical reaction needs a selected spectrum of an excitation light. However, although the spectral absorption of SiH.sub.4 usually occurs only in the wavelength region less than about 165 nm, the deposition occurs at a significant rate (several tens to several hundreds .ANG./min) with the ArF excimer laser having a wavelength of 193 nm or the KrF excimer laser having a wavelength of 248 nm. It is now believed that this wonder fact is caused by multiphoton absorption due to an extremely high density of photons in the laser beam. For example, it is thought that the ArF excimer laser is absorbed not as one-photon absorption (corresponding to a wavelength of 193 nm) but as two-photon absorption (96 nm) or three photon absorption (65 nm).
Actually, possibility of the multiphoton dissociation is strongly suggested by the following fact. The thin film is not deposited by ultraviolet radiation with a relatively lower photon density (e.g., ultraviolet with a wavelength of 185 nm emitted from a low pressure mercury lamp does not cause siH.sub.4 to deposite any thin film). Further, even with an excimer laser, there exist a threshold value of the radiation power density for the deposition of a thin film, and a power density of about several MW/cm.sup.2 is required.
In the meantime, photo CVD has been thought to be definitely different from plasma CVD in that photo CVD would not involve charged particles such as electrons and ions but involve only free radicals (i.e., excited reactive species without any charge) during deposition process.
In many cases of photochemical reactions due to the multiphoton absorption, however, the reactive gas absorbs an energy higher than ionization potential. As a matter of fact, two photons in an ArF excimer laser beam have an energy of about 13 eV which exceeds the ionization potential of SiH.sub.4. Further, in a photoionization experiment of SiH.sub.4 with synchrotron orbit radiation, the threshold energy for the photoionization is measured to be 11.5-12.0 eV. The photoionization cross section is maximized at about 13.4 eV, and then mainly SiH.sub.3.sup.+ and SiH.sub.2.sup.+ are produced.
As described above, when the multiphoton absorption occurs with light such as a laser beam, it is highly probable that ionization is involved in the photochemical reaction.